JPH11287883A - Nuclear fuel pellet, its production method, its fuel element and fuel assembly - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide nuclear fuel pellets having the same average concentration of Gd and high thermal conductivity by providing regions with high nuclear poison concentration such as Gd without generating microcracks in the pellets. SOLUTION: For nuclear fuel pellets 1 of UO2 and the like added by nuclear poison such as Gd2 O3 , regions with locally higher composition of nuclear poison concentration are scattered in the pellets 1 and all compositions are made to be fluorite type crystal structure or cubic crystal structure. Granulated powder containing higher nuclear poison than nuclear fuel pellet average concentration and consisting of composition making fluorite type crystal structure or cubic crystal structure after sintering and nuclear fuel powder are mixed, compression- formed and sintered in 1600 to 1800 deg.C. This is done in combustion gas having oxygen partial pressure.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a nuclear fuel pellet containing a nuclear poison such as Gd ₂ O ₃ and a method for producing the same.
A fuel element loaded with the nuclear fuel pellet; and a fuel assembly incorporating the fuel element.

[0002]

2. Description of the Related Art At present, Gd is used as a neutron absorbing material in nuclear fuel for light water reactors, and the concentration of Gd added tends to increase as the burnup of nuclear fuel for light water reactors increases.
Usually, the UO ₂ nuclear fuel pellet to which Gd is added is obtained by mechanically mixing UO ₂ powder and Gd ₂ O ₃ powder, compacting the mixture, and then sintering.

The mixed oxide of UO ₂ and Gd ₂ O ₃ is UO
₂ Compared sintering is low, the same sintering conditions, the density of the sintered nuclear fuel pellet, grain size be both smaller than UO ₂ sintered nuclear fuel pellets are generally known. It is known that when the additive concentration of Gd is high, fine cracks (microcracks) easily occur during sintering.

When the crystal grain size is small, the fission gas (FP)
Gas) has a disadvantage in that the diffusion distance of the gas decreases and the amount of FP gas released during combustion and the amount of swelling increase. Also, the presence of microcracks reduces the effective thermal conductivity of the nuclear fuel pellets, and thus increases FP gas emissions and swelling during combustion.

[0005] In order to increase the crystal grain size of the sintered nuclear fuel pellet to which Gd ₂ O ₃ is added and not to generate microcracks, it is necessary to improve the solid solution state of UO ₂ and Gd ₂ O _3. Conventionally, a method for producing nuclear fuel pellets having a uniform structure, that is, nuclear fuel pellets having a good solid solution state of UO ₂ and Gd ₂ O ₃ has been studied.

Among them, UO ₂ produced by the coprecipitation method
How to use the -gd ₂ O ₃ powder mixture, U in solution
And Gd are mixed and precipitated at the same time, so that a very uniformly mixed UO ₂ -Gd ₂ O ₃ mixed powder can be obtained, and the solid solution of UO ₂ and Gd ₂ O ₃ in the sintered nuclear fuel pellets can be obtained. Nuclear fuel pellets in an extremely good condition and having a uniform structure can be obtained.

However, this method has the disadvantages that the process is very complicated and expensive, and that a large amount of radioactive waste liquid contaminated with nuclear fuel material is generated during precipitation. Therefore, a method for producing a fuel pellet having a good solid solution state by using mechanically mixed UO ₂ -Gd ₂ O ₃ mixed powder (for example, JP-A-5-11088, JP-A-1-193691, Japanese Unexamined Patent Publication No. 2-242195) has been proposed.

In these methods, a method of sintering by adding a sintering aid is employed. It is also known that a higher solid solution state can be obtained by increasing the oxygen potential and promoting cation diffusion.

On the other hand, when Gd forms a solid solution in UO ₂ , Journa
As reported in l of Nuclear Materials, vol. 105 (1982), p. 201 et seq., it is known that the thermal conductivity is lowered by increasing the impurity concentration in UO ₂ .

[0010] This decrease in thermal conductivity results in an increase in the fuel center temperature and consequently the amount of FP gas release and swelling,
This leads to an increase in the pellet cladding interaction (PCI). Therefore, among the fuel elements constituting the fuel assembly, Gd ₂ O
_The maximum power experienced by a fuel element loaded with tri-added nuclear fuel pellets is usually designed to be lower.

Therefore, by suppressing the solid solution of Gd,
Methods for reducing the decrease in thermal conductivity of pellets (for example, JP-A-59-90082 and JP-A-60-231195) have been proposed. Japanese Patent Application Laid-Open No. 59-90082 discloses that UO ₂ particles and Gd ₂ particles during sintering are added by adding 1 μm to 20 μm Gd ₂ O ₃ particles coated with tungsten boride.
A method is described in which Gd is prevented from forming a solid solution by avoiding contact with O ₃ particles.

Japanese Patent Application Laid-Open No. 60-231195 discloses a method of adding Gd ₂ O ₃ particles of 1 mm or more to reduce the contact area between UO ₂ particles and Gd ₂ O ₃ particles during sintering.
A method for suppressing solid solution of d is shown.

[0013]

SUMMARY OF THE INVENTION However, Japanese Patent Application Laid-Open
According to the method of JP 60-231195 A, UO ₂ and Gd ₂
The reaction with O ₃ cannot be completely avoided and during sintering U
At the reaction interface between O ₂ and Gd ₂ O ₃ , a phase change accompanied by a volume change continues to occur, so that pores and cracks are generated near the interface. These pores and cracks cause a decrease in density, a decrease in thermal conductivity, and an increase in the release path of FP gas, so that the expected effects cannot be obtained.

An example of applying the same concept is an international conference (In
ternational Atomic Energy Agency, Technical Committ
ee Meeting on Advances in Pellet Technology for Im
proved Performance at High Burnup, Tokyo, Japan, 28 O
ctober-1 November 1996, Paper No.2-1 ) have been reported in, Gd ₂ O ₃ globules distributed nuclear fuel by sintering a mixture is heat-treated globules Gd ₂ O ₃ to UO ₂ powder I have pellets.

However, microcracks were not observed in this nuclear fuel pellet, but bubbles were gathered around the Gd ₂ O ₃ small spheres and were partially connected. This suggests that the improvement in thermal conductivity is small.

In the method disclosed in Japanese Patent Application Laid-Open No. 59-90082, the steps are complicated, the cost is high, the method is not industrial, and cracks are formed near the interface due to the difference in thermal expansion coefficient between tungsten boride and UO _2. This is expected to cause a decrease in thermal conductivity, and the expected effect cannot be obtained.

The present invention has been made in view of such a problem, and relates to an oxide sintered nuclear fuel pellet containing a nuclear poison such as Gd ₂ O ₃ , and more specifically, a nuclear fuel pellet such as Gd. The existence of a region with a concentration higher than the average nuclear fuel pellet concentration of the target poison prevents the decrease in the thermal conductivity of the nuclear poison such as Gd as compared with the conventional nuclear fuel pellet having the same average nuclear fuel pellet. It is an object of the present invention to provide a Gd ₂ O ₃ -added sintered nuclear fuel pellet, a method for producing the nuclear fuel pellet, a fuel element loaded with the nuclear fuel pellet, and a fuel assembly incorporating the fuel element.

[0018]

According to the first aspect of the present invention, a UO
_In a nuclear fuel pellet obtained by adding a nuclear poison such as Gd ₂ O ₃ to a nuclear fuel such as _{2 and} sintering, a region having a locally high nuclear poison concentration is dispersed in a matrix of the nuclear fuel pellet, In addition, any composition has a fluorite crystal structure or a cubic crystal structure.

The invention of claim 2 is characterized in that the high nuclear poison concentration region dispersed in the parent phase of the nuclear fuel pellet has an average diameter or an equivalent diameter of 50 μm or more. The invention according to claim 3 is that, among the elements constituting the high nuclear poison concentration region dispersed in the parent phase of the nuclear fuel pellet, the nuclear poison is Gd ₂ O ₃ , and Gd ₂
The O ₃ content is at most 60 wt%.

The invention according to claim 4 is characterized in that G is used for nuclear fuel such as UO _2.
A method for producing a nuclear fuel pellet to which a nuclear poison such as d ₂ O ₃ is added and sintered, the nuclear fuel pellet containing a nuclear poison higher than the average concentration of the nuclear fuel pellet, and having a fluorite-type crystal structure or a cubic crystal structure after sintering. The granulated or agglomerated particle powder having the composition as described above and the nuclear fuel powder are mixed at a predetermined mixing ratio, and the resulting mixture is compression-molded into a compression-molded product, which has a high sintering temperature in the range of 1600 to 1800 ° C. The compression molded body is sintered in a sintering gas having an oxygen partial pressure in which a nuclear poison concentration portion can have a fluorite crystal structure or a cubic crystal structure.

According to a fifth aspect of the present invention, a powder containing a nuclear poison higher than the average concentration of the nuclear fuel pellets and having such a composition as to have a fluorite-type crystal structure or a cubic structure after sintering is preformed, It is characterized by pulverizing and sieving to obtain strong aggregated particles capable of maintaining most of the granulated or aggregated particles when mixed with nuclear fuel powder.

According to a sixth aspect of the present invention, after the preforming,
Contains nuclear poisons higher than the average concentration by grinding and sieving,
And, in order to obtain a fluorite-type crystal structure or a cubic structure in any composition after sintering, when obtaining the granulated or agglomerated particle powder in the compression pre-forming step, the granulated or agglomerated particles during the compression pre-forming It is characterized in that compression preforming is performed at a pressure that gives a strength capable of deforming.

The invention of claim 7 is characterized in that nuclear poisons in the granulated or agglomerated particles containing a nuclear poison higher than the average concentration of the nuclear fuel pellets are mixed to such an extent that they can become a solid solution after sintering. I do.

The invention of claim 8 is characterized in that the aggregated particles containing a nuclear poison higher than the average concentration of the nuclear fuel pellets are already in a solid solution before sintering. The invention of claim 9 is characterized in that the average diameter or equivalent diameter of the granulated or agglomerated particle powder containing a nuclear poison higher than the nuclear fuel pellet average concentration is about 50 μm or more.

According to a tenth aspect of the present invention, the nuclear poison is Gd ₂
O ₃ , and the Gd ₂ O ₃ content in the high nuclear poison concentration region is at most 60 wt%. The invention of claim 11 is characterized in that the sintering gas is a mixed gas of CO and CO ₂ .

According to a twelfth aspect of the present invention, the nuclear poison is Gd ₂ O ₃
Using a granulated or agglomerated particle powder having a Gd ₂ O ₃ content of at most 30 wt% in a high nuclear poison concentration region and having an oxygen partial pressure equal to or higher than that of hydrogen gas having a dew point of −60 ° C. It is characterized by sintering in an atmosphere gas.

A thirteenth aspect of the present invention is a nuclear fuel element comprising a plurality of sintered nuclear fuel pellets loaded in a fuel cladding tube and enclosing both ends of the cladding tube. A region having a locally high poison concentration is dispersed and a fluorite-type crystal structure or a cubic structure is formed in any composition, or a high nuclear poison concentration dispersed in a matrix of the nuclear fuel pellet. The average diameter or equivalent diameter of the region is 50 μm or more, or the high nuclear poison concentration region dispersed in the parent phase of the nuclear fuel pellet has a Gd ₂ O ₃ content of at most 60 wt%. Features.

According to a fourteenth aspect of the present invention, in the fuel assembly, a plurality of sintered nuclear fuel pellets are loaded into a fuel cladding tube, and a fuel element or the like having both ends of the cladding tube sealed therein is incorporated. In the fuel element, a region composed of a composition having a locally high concentration of nuclear poison is dispersed in a matrix of the nuclear fuel pellet, and a fluorite-type crystal structure or a cubic structure is formed in any composition, or the nuclear fuel The average or equivalent diameter of the high nuclear poison concentration region dispersed in the parent phase of the pellet is 50 μm or more, or Gd _{2 of} the high nuclear poison concentration region dispersed in the parent phase of the nuclear fuel pellet.
A plurality of sintered nuclear fuel sintered pellets having a maximum O ₃ content of 60 wt% are loaded in the cladding tube.

[0029]

BEST MODE FOR CARRYING OUT THE INVENTION A nuclear fuel pellet according to the present invention
It is composed of a parent phase containing a nuclear toxin at a concentration lower than the average pellet concentration or a parent phase containing no nuclear toxin and a high nuclear toxin dispersed phase containing a higher concentration of nuclear toxin than the average pellet,
Both have a fluorite type crystal structure or a cubic structure close to it.

According to the nuclear fuel pellets of the present invention, since there is no microcrack, the decrease in thermal conductivity due to solid solution of nuclear poisons is small, and the content ratio of the parent phase having high thermal conductivity in the pellets is high. The effective thermal conductivity is higher than that of a conventional homogeneous solid solution nuclear fuel pellet having an average nuclear poison concentration of the pellet.

The method for producing nuclear fuel pellets according to the present invention is the method for producing nuclear fuel pellets obtained by adding a nuclear poison such as Gd ₂ O ₃ to a nuclear fuel such as UO _{2 and} sintering the fuel, wherein the average concentration of the nuclear fuel pellets is higher than the average concentration. A granulated or agglomerated particle powder containing a nuclear poison and having a composition such as to have a fluorite-type crystal structure or a cubic structure after sintering is mixed with a nuclear fuel powder at a predetermined mixing ratio, and compression-molded. The compression molding is performed at a sintering temperature in the range of 1600 ° C. to 1800 ° C. in a sintering gas having an oxygen partial pressure in which a high nuclear poison concentration portion can be a fluorite type crystal structure or a cubic crystal structure. The body is sintered to produce nuclear fuel pellets.

According to the method for producing nuclear fuel pellets of the present invention, the same fluorite-type crystal structure as that of the parent phase or a cubic crystal structure very similar to that of the parent phase is obtained even in a region containing a high concentration of nuclear poison. Due to sintering, the crystal structure is significantly different, and the occurrence of microcracks and the formation of another phase that hinders the growth of crystal grains can be suppressed. Nuclear fuel pellets containing poisons can be obtained.

For example, when the oxygen partial pressure in the sintering atmosphere gas is adjusted in the Gd ₂ O ₃ -added nuclear fuel pellet, in the sintering process, a region containing a high concentration of Gd ₂ O ₃ from the sintering gas becomes a fluorite type. Oxygen necessary for obtaining a crystal structure is supplied, and the nuclear fuel pellet has a structure having no phase boundary.

[0034] Therefore, without defects micro cracks after sintering exist, and the much lower concentration than Gd ₂ O ₃ concentration of nuclear fuel pellets average Gd ₂ O ₃
Nuclear fuel pellets having a large amount of a region containing only the same are obtained. By using the nuclear fuel pellets manufactured as described above, a decrease in thermal conductivity due to microcracks can be avoided.

In addition, the ratio of the parent phase having a high thermal conductivity is small because the decrease in thermal conductivity due to the solid solution of Gd is high, and it is more effective than the conventional homogeneous solid solution type nuclear fuel pellet having the same average nuclear poison concentration of the nuclear fuel pellet. The thermal conductivity increases, the increase in fuel center temperature is reduced, and consequently F
P gas release and swelling amount can be reduced.

By loading such a nuclear fuel pellet into a fuel cladding tube to form a fuel element, and incorporating this fuel element into a fuel assembly, the output of the fuel element containing a nuclear poison can be conventionally reduced. It can be set higher than that.

FIG. 1 shows a first example of a nuclear fuel pellet according to the present invention.
This nuclear fuel pellet 1 is made of U
The O ₂ mother phase is half distributed with globules dispersed phase of _{UO 2 -30wt% Gd 2 O 3} . Figure 2 is intended to also show the second embodiment, the nuclear fuel pellets 2 is a semi-distributed with globules dispersed phase of UO ₂ -50wt% Gd ₂ O ₃ to UO ₂ matrix phase.

V in FIGS. 1 and 2 represents the volume, and FIG.
Medium, VUO ₂ -50 wt% Gd ₂ O ₃ is UO ₂ -50 wt%
The volume of Gd ₂ O ₃ part, Vtotal represents the total volume,
VUO ₂ -50 wt% Gd ₂ O ₃ /Vtotal=0.22 in FIG.
8 shows that the volume ratio of UO ₂ -50 wt% Gd ₂ O ₃ part to the whole is 0.228. The same applies to FIG.

FIG. 3 shows a conventional nuclear fuel pellet 3,
O ₂ -10 wt% Gd ₂ O ₃ Uniform solid solution type. FIG.
FIG. 3 is a perspective view of FIG.

Table 1 shows the relative thermal conductivity of UO ₂ between the nuclear fuel pellet according to the present invention and the conventional nuclear fuel pellet. In addition, the pellet A of the present invention in the table is UO ₂ -60 wt.
% Gd ₂ O ₃ are small-sphere-dispersed nuclear fuel pellets. The pellet B of the present invention corresponds to the nuclear fuel pellet 1 of the first embodiment, and the conventional pellet has almost completely dissolved Gd shown in FIG. It corresponds to UO ₂ -10 wt% Gd ₂ O ₃ nuclear fuel pellet 3.

These values are set in the Journal of Nuclear Mater
ials, Vol. 231 (1996), from page 29 onwards.
O ₂ -Gd ₂ O ₃ of the thermal conductivity of the pellets (solid solution) Gd
₂ O ₃ concentration dependence formula, and High Temperature -High
This is a value calculated using the thermal conductivity formula in the case where particles having different thermal conductivity are dispersed in a matrix, as shown in Pressures Vol. 13 (1981), page 649 or later.

[0042]

[Table 1] As is clear from Table 1, it is recognized that the thermal conductivity of the nuclear fuel pellet according to the present invention is 10% to 50% larger than that of the conventional pellet.

Next, FIGS. 4 and 5 show nuclear fuel pellets according to the present invention and Examples 1 to 10 of a method for producing the same.
This will be described with reference to FIG. Example 1 As shown in FIG. 4, the Gd ₂ O ₃ concentration was 30 wt.
% Of the UO ₂ powder 4 and the Gd ₂ O ₃ powder 5 were mixed 6 by a ball mill to obtain a uniform mixed powder, which was pressed at 2.2 t / cm ² and compression preformed 7. Thereafter, the mixture is pulverized and sieved 8 to obtain UO ₂ -Gd ₂ O ₃ granulated powder 9 having a size of about 50 μm or more.

This granulated powder 9 was prepared with a Gd ₂ O ₃ concentration of 10 wt%.
In addition to the UO ₂ powder 4, the granulated powder 9 is mixed for 10 minutes with a V-blender that is not pulverized, and the UO ₂ -10 wt% G
obtain d ₂ O ₃ powder mixture. This was pressed at 2.2 t / cm ² and compression-molded to obtain a molded body 12 having a relative density of about 53% with respect to the theoretical density, and then heated at 1740 ° C. in an N ₂ + 13% H ₂ atmosphere having a dew point of 10 ° C. Sintering 14 for 4 hours. As a result, a sintered body 15 having a relative density to the theoretical density of about 94% and a crystal grain size of about 8 μm was obtained. This sintered body 15 is made of UO ₂ -10 wt% Gd ₂ O
It was a nuclear fuel pellet composed of the small sphere dispersed phase ₃ and the UO ₂ matrix.

[Embodiment 2] In the same manner as in Embodiment 1, Gd
_A predetermined amount of UO ₂ powder 4 so that the ₂ O ₃ concentration is 30 wt%
And Gd ₂ O ₃ powder 5 and a small amount of alumina silica mixed powder 16
Are mixed in a ball mill 6 to obtain a uniform mixed powder.
After pressing at 2.2t / cm ² and compression preforming 7, pulverizing
The mixture is sieved 8 to obtain a granulated powder 9 of about 50 μm or more. This granulated powder 9 is mixed with UO ₂ so that the Gd ₂ O ₃ concentration becomes 10 wt%.
In addition to powder 4, granulated powder 9 is V-blender that is not crushed.
Mixing 10 for 30 minutes to obtain UO ₂ -10 wt% Gd ₂ O ₃ mixed powder. This was compression-molded 11 at 2.2 t / cm ² to obtain a molded body 12 having a relative density of about 52% with respect to the theoretical density.
Sintering at 1740 ° C for 4 hours in N ₂ + 13% H ₂ atmosphere
14 As a result, the relative density to the theoretical density is about 94
%, And a sintered body 15 having a crystal grain size of about 20 μm was obtained.

Example 3 As in Example 1, Gd ₂ O ₃
A predetermined amount of UO ₂ powder 4 and Gd so that the concentration is 30 wt%
₂ O ₃ powder 5 was mixed with a ball mill 6 to obtain a uniform mixed powder, which was pressed at 0.5 t / cm ² to pre-compression molding 7.
Then, the mixture is pulverized and sieved 8 to obtain a granulated powder 9 having a size of 60 μm or more. The granulated powder 9 is added to the UO ₂ powder 4 so that the Gd ₂ O ₃ concentration becomes 10 wt%, and the granulated powder 9 is mixed for 10 minutes with a V-blender in which the granulated powder 9 is not pulverized to obtain UO ₂ -10 wt% Gd ₂ O.
_3. Obtain a mixed powder. This was compression-molded 11 at 2.2 t / cm ² to obtain a molded body 12, and this molded body 12 was cooled with N ₂ +13 having a dew point of 10 ° C.
Sintering was carried out at 1740 ° C. for 4 hours in a% H ₂ atmosphere. As a result, a sintered body 15 having a relative density to the theoretical density of about 94% and a crystal grain size of about 10 μm was obtained.

Embodiment 4 As shown in FIG. 5, Gd ₂ O
₃ Predetermined amount of UO ₂ powder 4 and G so that the concentration becomes 50 wt%
The d ₂ O ₃ powder 5 and each were dissolved in nitric acid 17 and dripped, precipitated and aged 19 by sol-gel method, and roasted, reduced and sieved.
After passing through 20, uniform aggregated particles 21 of about 50 μm or more are obtained. The aggregated particles 21 are mixed with UO so that the Gd ₂ O ₃ concentration becomes 10 wt%.
_{2 In} addition to the powder 4, the mixture 10 is mixed with a V blender in which the aggregated particles 21 are not pulverized to obtain a UO ₂ -10 wt% Gd ₂ O ₃ mixed powder. This was compression-molded 11 at 2.2 t / cm ² to obtain a molded body 12, and the molded body 12 was sintered at 1740 ° C. in an atmosphere of 40 wt% CO + 60% CO ₂ to obtain a sintered body 15. To obtain sintered nuclear fuel pellets.

A UO containing 60 wt% or more of Gd ₂ O ₃
Examination of the relationship between the sintering atmosphere and the crystal structure of the _two powders revealed that under conditions where UO ₂ did not form a U ₄ O ₉ phase having a different crystal structure, the Gd ₂ O ₃ high-concentration portion had a fluorite type. It turned out that it did not become a crystal structure. In FIGS. 4 and 5, a lubricant 23 is added as needed in the mixing step 10 before the compression molding 11, but the molded body 12 is degreased 13 before sintering 14.

Example 5 In Example 4 shown in FIG. 5, granulated powder and agglomerated particles produced by ball mill mixing + pressing and a sol-gel method were used, but another mixing method for uniformly mixing the powder was used. The powder obtained by mixing and granulating according to the above method may be used, or particles obtained by forming a solid solution in advance by sintering and then pulverizing the solid solution may be used.

[Embodiment 6] In Examples 1 to 4, Gd ₂ O ₃ has been described as an example of a nuclear poison, but other nuclear poisons such as Er and Dy can be expected to have the same effect. Since sintering has similar chemical properties to Gd, sintering conditions and the like can be determined based on a similar idea.

[Embodiment 7] In Embodiments 1 to 4, Gd, U, O, and
Although an example using only Al and Si has been described, by using agglomerated particles that generate the same degree of sintering as the parent phase during sintering and do not significantly differ in crystal structure, micro-cracks or crystals during sintering are used. The formation of a reaction phase that inhibits grain growth, the formation of continuous pores, and the like can be avoided.

For example, by adding Y ₂ O ₃ in ZrO ₂ , a nuclear toxic substance such as Gd ₂ O ₃ can be stably maintained even at a high temperature so that a cubic crystal can be stably maintained from room temperature to a high temperature. By adding an additive that can be maintained as above, the same effects as in Examples 1 to 4 can be obtained.

Embodiment 8 In Embodiments 1 to 4, the CO—CO ₂ mixed gas and the H ₂ O
Has been described in the example of N ₂ + 13% H ₂ gas to which sintering is added. However, since the sintering behavior is affected by the oxygen potential of the atmosphere gas which equilibrates with the solid phase during sintering, other atmospheres having the same oxygen potential are used. The same effect can be obtained by using a gas.

[0054] In Example 9 Examples 1 to 4 has been described in an example of using a mixed agglomerated particles comprising a Gd ₂ O ₃ up to 50 wt%, including Gd ₂ O ₃ higher concentrations Even when the mixed agglomerated particle powder is used, the same effects as those of the present invention can be obtained because the basic behavior during molding and sintering is the same.

However, U containing 60% by weight or more of Gd ₂ O ₃
When the relationship between the sintering atmosphere and the crystal structure of the O ₂ powder was examined, under the condition that UO ₂ did not form a U ₄ O ₉ phase having another crystal structure, the Gd ₂ O ₃ high concentration portion was fluorite. It was observed that no type crystal structure was formed.

[Embodiment 10] In the embodiments 1 to 4, the example of the UO ₂ powder as the raw material powder constituting the mother phase has been described, but any one of UO ₂ , PuO ₂ , and ThO ₂ , or a mixture thereof, may be used. Mixtures, or even powders containing nuclear toxicants below the average pellet concentration, can be used. According to this embodiment, there is no change in the basic phenomena of the behavior at the time of molding and the sintering behavior of the powder, and since the thermal conductivity is higher than that of the conventional example, the same effects as in Examples 1 to 4 can be obtained. .

Next, Reference Examples 1 to 3 for comparison with Examples 1 to 10 will be described below. [Reference Example 1] Gd ₂ O ₃ powder 5 was calcined at 1500 ° C. to obtain a stable monoclinic at a high temperature according to FIG. 4, then pressed at 3.1 t / cm ² , compression preformed, and pulverized. · after sieving, the ones subjected to spheroidizing treatment 22 Gd ₂ O ₃ concentration is added to the UO ₂ powder 4 to be 10 wt%, were mixed 10 V blender granulated particles 9 are not crushed, UO ₂ obtaining -10wt% Gd ₂ O ₃ powder mixture.

This was pressed at 2.2 t / cm ² and subjected to compression molding 11 to obtain a molded body 12 having a relative density of about 53% with respect to the theoretical density, and then, in a N ₂ + 13% H ₂ atmosphere at a dew point of 10 ° C. 1740
Sintered at 14 ° C. for 4 hours. As a result, a sintered body 15 having a relative density to the theoretical density of about 95% and a crystal grain size of about 15 μm was obtained.

However, the sintered body 15 obtained as the reference example, that is, the Gd ₂ O ₃ region and the U
Many continuous pores and long and narrow pores were present at the boundary of the O ₂ region, and a favorable structure could not be obtained. This means that even when the molding pressure is 1.3 t / cm ² , the sintering density is about
At 95%, the tissues were similar.

Reference Example 2 According to Reference Example 1, UO
_{When the} powder 4 and the Gd ₂ O ₃ powder 5 were mixed, a small amount of alumina-silica mixed powder 16 was added, and sintering was performed under the same conditions. The relative density with respect to the theoretical density was about 93%, and the crystal grain size was about A 20 μm sintered body 15 was obtained. However, this sintered body 1
5, that is, at the boundary between the Gd ₂ O ₃ region and the UO ₂ region of the nuclear fuel pellet, more continuous pores and elongated pores were present, and a favorable structure could not be obtained.

[Reference Example 3] Gd ₂ O ₃ powder was pressed, pulverized and spheroidized, and then heat-treated at 1700 ° C. to obtain a monoclinic crystal. A sintered body was prepared with the addition of the alumina-silica mixed powder, but in each case, a larger number of continuous pores were present at the boundary between the Gd ₂ O ₃ region and the UO ₂ region, and a preferable structure was not obtained. .

Next, an embodiment of the fuel element according to the present invention will be described with reference to FIG. FIG. 6A is a perspective view, partially cut away, showing an example of an embodiment of the fuel element 24 according to the present invention, and FIG. 6B is a perspective view showing an example of a fuel assembly 25 incorporating the fuel element 24. Indicated by.

The fuel element 24 shown in FIG. 6 (a) has a plurality of nuclear fuel pellets 27 stacked and loaded in a zirconium-based long cylindrical fuel cladding tube 26, and upper and lower ends of the fuel cladding tube 26. Is sealed by an upper end plug 28 and a lower end plug 29.

A plenum spring 30 is provided between the upper end plug 28 and the upper end of the nuclear fuel pellet 27. The plenum spring 30 prevents the nuclear fuel pellet 27 from moving up and down.

Here, the nuclear fuel pellets 27 were used in Examples 1 to 10.
And manufactured as follows. That, Gd ₂ nuclear fuel, such as UO ₂
In a nuclear fuel pellet to which a nuclear poison such as O ₃ has been added and sintered, a region having a composition in which the concentration of the nuclear poison is locally high is dispersed in the nuclear fuel pellet, and the fluorite-type crystal structure or It has a cubic structure.

The average diameter or equivalent diameter of highly nuclear poisons dispersed in the parent phase of nuclear fuel pellets is 50 μm or more. Further, among the elements constituting the high nuclear poison concentration region dispersed in the parent phase of the nuclear fuel pellet, the nuclear poison is Gd2.
O3, and the content of Gd2 O3 in the high nuclear poison concentration region is at most 60 wt%.

According to the present embodiment, the fuel element can reduce the fuel temperature when compared at the same output as compared with the conventional example. In other words, when a design that allows the fuel temperature to rise to the same fuel temperature, that is, a design that maintains the same fuel behavior, a higher output can be allowed.

For example, when the nuclear fuel pellets manufactured according to the first embodiment are loaded into the fuel cladding tube 26 to constitute the fuel element 24, the temperature rise from the surface of the nuclear fuel pellets 27 to the center of the fuel is compared with the same output. , Compared to the conventional example
It can be reduced by 10% or more. In other words, the output can be increased by 10% or more.

Next, an embodiment of a fuel assembly according to the present invention will be described with reference to FIG. The fuel assembly 25 shown in FIG. 6 (b) incorporates and fixes the upper and lower ends of the fuel element 24 shown in FIG. 6 (a) into the upper tie plate 31 and the lower tie plate 32. Is held by the spacer 33. The outside of the fuel assembly 25 is surrounded by a rectangular tubular channel box (not shown). In FIG. 6B, reference numeral 34 denotes a channel fastener, which is fixed by the lower tie plate 32 and the lower end plug 29. According to the present embodiment, by incorporating the fuel element 24 in FIG. 6A, it is possible to reduce the suppression of the output due to the low thermal conductivity.

[0070]

According to the fuel pellets containing a nuclear poison such as Gd ₂ O ₃ according to the present invention, a decrease in thermal conductivity due to microcracks can be avoided, and a solid solution of a nuclear poison such as Gd can be prevented. By reducing the decrease in thermal conductivity,
The increase in the fuel temperature during the irradiation is reduced, and as a result, the FP gas release and the swelling amount can be reduced. Further, when the fuel behavior equivalent to the conventional example is allowed, the output can be set higher than the conventional example.

[Brief description of the drawings]

FIG. 1 is a perspective view schematically showing a model of a nuclear fuel pellet according to a first embodiment of the present invention.

FIG. 2 is a perspective view schematically showing a first embodiment of the nuclear fuel pellet according to the present invention in a replicated manner.

FIG. 3 is a perspective view schematically showing a conventional nuclear fuel pellet by copying.

FIG. 4 is a process chart for explaining a first embodiment of the nuclear fuel pellet manufacturing method according to the present invention.

FIG. 5 is a process chart for explaining a second embodiment of the method for producing nuclear fuel pellets according to the present invention.

FIG. 6 (a) is a perspective view showing a partial cross section for explaining one embodiment of a fuel element according to the present invention, and FIG. 6 (b) is a view for explaining one embodiment of a fuel assembly according to the present invention. FIG.

[Explanation of symbols]

1, 2 nuclear fuel pellets of the present invention, 3 ... conventional nuclear fuel pellet, 4 ... UO ₂ powder, 5 ... Gd ₂ O ₃ powder, 6 ... mixing, 7 ... compression preforming, 8 ... divided ground and screened, 9 ... Granulated powder, 10 mixed, 11 compaction, 12 compact, 13 degreasing,
14 ... Sintered, 15 ... Sintered body, 16 ... Alumina silica mixed powder,
17: dissolution, 18: mixing, 19: dripping / precipitation / aging, 20: roasting / reduction / sieving, 21: aggregated particles, 22: spheroidizing treatment, 23 ...
Lubricant, 24 fuel element, 25 fuel assembly, 26 fuel cladding tube, 27 nuclear fuel pellet, 28 upper plug, 29 lower plug, 30 plenum spring, 31 upper tie plate,
32: Lower tie plate, 33: Spacer, 34: Channel fastener.

 ──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Mutsumi Hirai 2163, Narita-cho, Oarai-machi, Higashiibaraki-gun, Ibaraki Prefecture Within Japan Nuclear Fuel Development Co., Ltd. (72) Inventor Yuki 2163, Narita-cho, Oarai-machi, Higashiibaraki-gun, Ibaraki Within Japan Nuclear Fuel Development Co., Ltd. (72) Inventor Chihiro Mizutani 2163, Narita-cho, Oarai-machi, Higashiibaraki-gun, Ibaraki Prefecture Within Japan Nuclear Fuel Development Co., Ltd. (72) Masaki Amaya, 2163, Narita-machi, Oarai-machi, Higashiibaraki-gun, Ibaraki Within Japan Nuclear Fuel Development Co., Ltd. (72) Inventor Yasushi Yanai 2163, Narita-cho, Oarai-machi, Higashiibaraki-gun, Ibaraki Prefecture Within Japan Nuclear Fuel Development Co., Ltd. (72) Hiroshi Masuda 2163, Narita-cho, Oarai-machi, Higashiibaraki-gun, Ibaraki Address Japan Nuclear Fuel Development Co., Ltd.

Claims (14)

[Claims] 1. A nuclear fuel pellet obtained by adding a nuclear poison such as Gd ₂ O ₃ to a nuclear fuel such as UO _{2 and} sintering the same, wherein the concentration of the nuclear poison in the parent phase of the nuclear fuel pellet is locally high. Nuclear fuel pellets characterized by having a dispersed region and a fluorite-type crystal structure or a cubic structure in any composition. 2. An average or equivalent diameter of the high nuclear poison concentration region dispersed in the parent phase of the nuclear fuel pellet is 50 μm.
The nuclear fuel pellet according to claim 1, wherein: 3. The high nuclear poison concentration region among the elements constituting the high nuclear poison concentration region dispersed in the parent phase of the nuclear fuel pellet, the nuclear poison is Gd ₂ O ₃ , and the high nuclear poison concentration region Gd
_2. The nuclear fuel pellet according to claim 1, wherein the content of ₂ O ₃ is at most 60 wt%. 4. A method for producing a nuclear fuel pellet obtained by adding a nuclear poison such as Gd ₂ O ₃ to a nuclear fuel such as UO _{2 and} sintering, wherein the nuclear poison contains a nuclear poison higher than the average concentration of the nuclear fuel pellet. A granulated or agglomerated particle powder having a composition such as to have a fluorite-type crystal structure or a cubic structure is mixed with a nuclear fuel powder at a predetermined mixing ratio, and the resulting mixture is compression-molded into a compression-molded body. At a sintering temperature in the range of ° C., characterized in that the high nuclear poison concentration portion is sintered in a sintering gas having an oxygen partial pressure capable of forming a fluorite-type crystal structure or a cubic structure. Of producing nuclear fuel pellets. 5. A powder containing a nuclear poison higher than the average concentration of the nuclear fuel pellets and having a composition such that a fluorite-type crystal structure or a cubic crystal structure is obtained after sintering.
5. The method for producing nuclear fuel pellets according to claim 4, wherein pulverized and sieved to obtain strong aggregated particles capable of maintaining most of the granulated or aggregated particles when mixed with nuclear fuel powder. 6. Granulated or agglomerated particles containing a nuclear poison higher than the average concentration by pulverization and sieving after said preforming, and having a composition such that a fluorite-type crystal structure or a cubic crystal structure is obtained after sintering. 6. The method for producing nuclear fuel pellets according to claim 5, wherein, when obtaining the powder, compression preforming is performed under such a pressure that the granulated or agglomerated particles are deformed during molding. 7. A nuclear poison in a granulated or agglomerated particle containing a nuclear poison higher than the nuclear fuel pellet average concentration is mixed to such an extent that it can become a solid solution after sintering. A method for producing nuclear fuel pellets according to claim 6. 8. The method for producing nuclear fuel pellets according to claim 4, wherein the aggregated particles containing a nuclear poison higher than the average concentration of the nuclear fuel pellets are already in a solid solution before sintering. . 9. The nuclear fuel according to claim 4, wherein the average diameter or the equivalent diameter of the granulated or agglomerated particle powder containing a nuclear poison higher than the nuclear fuel pellet average concentration is about 50 μm or more. Method for producing pellets. 10. The nuclear poison is Gd ₂ O ₃ , and the Gd ₂ O ₃ content in the high nuclear poison concentration region is at most 60 wt%.
The method for producing nuclear fuel pellets according to claim 4, wherein: 11. The method for producing nuclear fuel pellets according to claim 4, wherein said sintering gas is a mixed gas of CO and CO ₂ . 12. The method according to claim 12, wherein the nuclear poison is Gd ₂ O ₃ , and a granulated or agglomerated particle powder having a Gd ₂ O ₃ content of at most 30 wt% in a high nuclear poison concentration region, and having a dew point of −60 ° C. 10. The method for producing nuclear fuel pellets according to claim 4, wherein sintering is performed in an atmosphere gas having an oxygen partial pressure equal to or higher than that of hydrogen gas. 13. A nuclear fuel element comprising a plurality of sintered nuclear fuel pellets loaded in a fuel cladding tube and sealing both ends of the cladding tube, wherein the nuclear fuel pellets have a local concentration of nuclear poison in the pellets. The region having a high composition is dispersed, and has a fluorite-type crystal structure or a cubic structure in any composition, or the average diameter of the high nuclear poison concentration region dispersed in the parent phase of the nuclear fuel pellet or Gd having an equivalent diameter of 50 μm or more or a high nuclear poison concentration region dispersed in the parent phase of the nuclear fuel pellet.
A fuel element characterized in that it has a content of ₂ O _{3 of} at most 60 wt%. 14. A fuel assembly in which a plurality of sintered nuclear fuel pellets are loaded into a fuel cladding tube and a fuel element or the like in which both ends of the cladding tube are sealed is incorporated, wherein the fuel element is the nuclear fuel In the parent phase of the pellet, the region consisting of a composition having a locally high nuclear poison concentration is dispersed, and any composition has a fluorite-type crystal structure or a cubic crystal structure, or the parent phase of the nuclear fuel pellet Average diameter or equivalent diameter of high nuclear poison concentration area dispersed in 50μ
m or more, or a plurality of sintered nuclear fuel sintered pellets having a Gd ₂ O ₃ content of at most 60 wt% in a high nuclear poison concentration region dispersed in the parent phase of the nuclear fuel pellets. A fuel assembly comprising a cladding tube.